1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method for driving an LCD device, and more particularly, to an In-Plane-Switching (IPS) LCD device and a method for driving an IPS LCD device.
2. Description of the Related Art
In general, a liquid crystal display (LCD) device is formed by attaching a thin film transistor (TFT) array substrate and a color filter (CF) substrate together to face each other with a certain cell gap therebetween, and filling the cell gap with a liquid crystal material. A plurality of gate lines arranged at regular intervals along a horizontal direction and a plurality of data lines arranged at regular intervals along a longitudinal direction are formed on the TFT array substrate to intersect each other, thereby forming pixel regions each having a switching device and a pixel electrode at every intersection of the gate and data lines. In addition, red, green and blue color filters corresponding to the pixel regions are formed on the CF substrate, and a black matrix for preventing color interference of light passing through the pixel regions is formed in a matrix configuration that encompasses an outer edge of the color filters. Furthermore, a common electrode for supplying an electric field in conjunction with pixel electrodes in the pixel regions to the liquid crystal material.
Twisted nematic (TN) liquid crystal material is commonly used in LCD devices, and is driven by a vertical electric field formed between the pixel electrode and the common electrode. Accordingly, the TN liquid crystal material varies light transmittance according to a viewing angle of an observer. Thus, use of the TN liquid crystal material is limited for use in large-sized LCD devices. For example, since light transmittance is symmetrical along a horizontal direction and is asymmetrical along a vertical direction, an image is inverted along the vertical direction, thereby narrowing the viewing angle of the observer. In order to solve such problems, an IPS liquid crystal display device in which a liquid crystal material is driven by a horizontal electric field has been proposed.
The IPS LCD device may improve angular field characteristics, such as contrast, gray inversion, and color shift, in order to obtain a wide angular viewing field, as compared to the LCD device in which the liquid crystal material is driven by the vertical electric field. Accordingly, the IPS LCD device is commonly used in the large-sized LCD devices.
FIG. 1A is a schematic plan view of a TFT array substrate of an IPS LCD device according to the related art. In FIG. 1A, a plurality of gate lines (G1˜Gn) are arranged parallel to one another along a horizontal direction and a plurality of data lines (D1˜Dm) are arranged parallel to one another along a longitudinal direction. Accordingly, the gate lines (G1˜Gn) and the data lines (D1˜Dm) intersect at right angles, and a pixel region P1 is defined at each of the intersections. In order to control image information supplied to a pixel electrode 11, a switching device, such as a TFT TFT1, is provided at each of the pixel regions P1.
Although not shown, gate electrodes of each of the TFTs TFT1 are connected to the gate lines (G1˜Gn), source electrodes are connected to data lines (D1˜Dn), and drain electrodes are connected to the pixel electrode 11 in the pixel region P1. Accordingly, when scan signals of the LCD device are sequentially supplied to the gate lines (G1˜Gn), the TFTs TFT1 are sequentially turned ON by the gate lines (G1˜Gn). Thus, an electric conduction channel is formed between the source electrode and the drain electrode of each of the TFTs TFT1 that are turned ON by the gate lines (G1˜Gn), and the electric conduction channel supplies image information supplied to the source electrode of the TFTs TFT1 through the data lines (D1˜Dn), to the drain electrode. Since the drain electrode is connected to the pixel electrode 11, the image information is supplied to the pixel electrode 11. At least one of the pixel electrodes 11 in the pixel region P1 is patterned along a direction parallel to the data lines (D1˜Dn).
In the pixel region P1, the common electrode 13 formed parallel to the pixel electrode 11, which corresponds to the pixel electrode 11, generates a horizontal electric field together with the pixel electrode 11, thereby driving the liquid crystal material using an in-plane switching method. Similarly, like the pixel electrode 11, at least one of the common electrode 13 in the pixel region P1 is patterned.
A common voltage is supplied to the common electrode 13 formed in the pixel region P1 through common voltage lines (Vcom1˜Vcomm), the common voltage lines (Vcom1˜Vcomn) that are arranged parallel to the gate lines (G1˜Gn). In addition, one side of each of the common voltage lines is electrically connected to one side of each of the gate lines, thereby supplying the same common voltage to every pixel electrode 11.
Since the pixel electrode 11 is electrically connected to a storage capacitor (not shown), the image information supplied to the pixel electrode 11 is charged in the storage capacitor during a turn-ON period of the TFT TFT1, in which scan signals are supplied. Accordingly, the charged image information maintains a driving of the liquid crystal material by being supplied to the pixel electrode 11 during a turn-OFF period of the TFT TFT1, in which the scan signal are not supplied.
FIG. 1B is a schematic plan view of an equivalent circuit of pixels of the TFT array substrate of FIG. 1A according to the related art. In FIG. 1B, a pixel region P1 includes a TFT TFT1 having a gate electrode connected to a gate line (G1˜Gn) and a source electrode connected to a data line (D1˜Dn). In addition, a parasitic capacitor Clc is formed due to a capacitance of the liquid crystal material and a storage capacitor Cst is formed that both are connected in parallel between a drain electrode and the common voltage lines (Vcom1˜Vcomm) of the TFT TFT1.
When an electric field is continuously supplied to the liquid crystal material, the liquid crystal material deteriorates, thereby causing afterimages by a DC voltage component. Accordingly, in order to prevent deterioration of the liquid crystal material and to eliminate the DC voltage component, a positive (+) voltage and a negative (−) voltage of the image information are repeatedly supplied on the basis of the common voltage. Such a driving method is commonly called an inversion driving method.
Among the different types of inversion driving methods, there are a frame inversion driving method in which a polarity of image information is inverted by a unit of frame and then supplied, a line inversion driving method in which a polarity of image information is inverted by a unit of the gate line and then supplied, and a dot inversion driving method in which a polarity of image information is inverted by pixels adjacent to each other and then supplied, and also inverted by a unit of the frame of the image and then supplied. The dot inversion driving method of the above inversion driving methods may restrict image distortion, such as flicker or cross talk more effectively than the other inversion driving methods, thereby producing quality images.
FIG. 2 is a schematic diagram of voltage waveforms of a dot inversion method according to the related art. In FIG. 2, a common voltage (Vcom) is maintained as a DC voltage, and scan signals are sequentially supplied to gate lines in every frame. A positive polarity and a negative polarity of the image information (VDATA) are inverted by pixels adjacent to each other based on a common voltage and then supplied, and are also inverted by a unit of frame based on the common voltage and supplied.
During a turn-ON period in which the scan signals (VG1˜VG3) are supplied having high electric potential, image information (VDATA) supplied to the pixel electrode is charged in the storage capacitor and has a pixel voltage (Vp) waveform. In addition, when the scan signals (VG1˜VG3) are transited to a low electric potential, the pixel voltage (Vp) drops due to coupling of a parasitic capacitor according to overlap of the gate electrode and the drain electrode of the thin film transistor. The dropped amount of the pixel voltage (Vp) is commonly referred to as a range of fluctuation (ΔVp) of the pixel electrode.
During a turn-OFF period of the TFT, in which the scan signals (VG1˜VG3) are supplied with a low electric potential, the pixel voltage (Vp) charged in the storage capacitor is continuously supplied to the pixel electrode to maintain a driving of the liquid crystal material.
A voltage difference (VDATA-Vcom) obtained by subtracting the common voltage (Vcom) from the image information (VDATA) is commonly defined as a liquid crystal driving voltage (Vcel). Thus, in order that the liquid crystal driving voltage (Vcel) drives the liquid crystal material, the image information (VDATA) should be supplied with a voltage level more than, the common voltage (Vcom). However, this causes an increase in power consumption.
In addition, a magnitude of the liquid crystal driving voltage (Vcel) is dependent upon the image information because the common voltage (Vcom) is fixed at the specific level. In order to form a high electric field in the liquid crystal material, a source integrated circuit having a high output voltage should be used.
Furthermore, in the IPS LCD device, if an interval between a pixel electrode and a common electrode increases in order to obtain a high aperture ratio, a higher driving voltage is required in order to obtain a required brightness, thereby greatly increasing power consumption.